Devices for optical dosage measurement are required for the measurement of quantities of light or of the time integral of the product of exposure intensity and exposure duration, and for the production of an electrical signal corresponding to the measured quantity. Such "optical dosemeters" can be used in photoelectric measuring devices and control units, phototimers, optical storage devices, ergometers and power meters for intermittent or continuous operation.
It is known, for example from the publication "Sov. Phys. Semicond." Volume 10, No. 2, February 1976, pages 128-143, that in many semiconductors there is photoconductivity with very slow relaxation, i.e. that the increase in conductivity (photoconductivity) produced by incidence of optical radiation persists for a relatively long time even when the radiation is terminated. At sufficiently low operating temperatures the relaxation time can become practically infinite and reference is made in these and comparable cases to a "stored photoconductivity".
The effect of stored photoconductivity has hitherto not been exploited in optical dosage measurement, presumably due to the fact that there is no fully satisfactory explanation for the effect and it is therefore not known how to achieve the kind of adequate reproducibility which is essential for industrial use.